leetcode216. 组合总和 iii

class Solution {
    public List<List<Integer>> combinationSum3(int k, int n) {
        List<List<Integer>> res =  new ArrayList<>();
        List<Integer> list = new ArrayList<>();
        dfs(res, list, n,k,0,0);
        return res;
    }

    public void dfs(List<List<Integer>> res, List<Integer> list, int n, int k, int sum, int num) {
        if(sum > n) return;
        if (list.size() == k){
            if(sum == n) {
                res.add(new ArrayList<>(list));
            }
            return;
        }
        for (int i = num+1; i <= 9; i++) {
            list.add(i);
            dfs(res, list, n, k, sum+i, i);
            list.remove(list.size()-1);
        }

    }
}